The kidney plays a central role in the maintenance of sodium balance. Micropuncture studies of the factors controlling renal salt and water excretion is reduced to values less than 10 percent of those observed in awake animals. Our initial investigations have shown that the cause of the reduced salt and water excretion following surgery is a marked fall in plasma volume from awake animal values (red cell volume remaining unchanged). Correction of this intravascular hypovolemia by isoncotic plasma repletion restored salt and water excretion to values appropriate to the dietary history of the animal. Using plasma volume repletion as a standard procedure following micropuncture surgery the objectives of this proposal include 1) a study of the nephron site(s) and mechanism(s) of altered reabsorption of salt and water in response to alterations in dietary salt intake, 2) a reevaluation of the mechanism(s) of "escape" from the salt retaining effects of the hormone deoxycorticosterone (DOCA), and 3) an examination of glomerular dynamics and salt and water reabsorption in early stages of hypertension together with an examination of the site of action of various antihypertensive agents. BIBLIOGRAPHIC REFERENCES: Maddox, D.A. and B.M. Brenner. Glomerular filtration of fluid and macromolecules. The renal response to injury. The Annual Review of Medicine 28:91-102, 1977.